1. Field of the Invention
This invention relates to a cathode electrode structure for a sodium sulphur cell.
In a sodium sulphur cell, a solid electrolyte material separates molten sodium, forming the anode, from a sulphur/polysulphide cathodic reactant. In such a cell the solid electrolyte is a material, such as betaalumina, which conducts sodium ions. On discharge of the cell, the sodium gives up electrons at the anodic interface of the solid electrolyte and the sodium ions pass through the solid electrolyte into the cathode adjacent the electrolyte. In the cathodic region, these sodium ions have to combine with sulphide ions to form a sodium polysulphide. The electrons pass through the sodium to the anode current collector and thence around an external circuit to a cathode current collector, e.g. a carbon or graphite tube or rod, in the cathodic reactant. The electrons must pass from this cathode current collector to the region of the cathode adjacent the surface of the solid electrolyte where they react with the sulphur to form sulphide ions. The sulphide ions and sodium ions form a polysulphide. The electronic conductivity of molten sulphur is low and hence it is the practice to pack the cathodic region with a fibrous carbon or graphite material which provides the required electronic conductivity.
2. Prior Art
Carbon or graphite has been used for this packing because of the highly corrosive nature of the cathodic reactant comprising molten sulphur and sodium polysulphides, which reactant, when the cell is in operation, is typically at a temperature of the order of 350.degree.. Metals such as stainless steel are corroded in this environment. The carbon or graphite may be in the form of loose fibres or the fibres may be in the form of a felt or a woven cloth; another form of carbon which has been employed is reticulated vitreous carbon. The carbon or graphite material has to form a matrix, through which the liquid cathodic reactant can move. The polysulphides formed by the electrochemical reaction have to be transferred away from the neighbourhood of the electrolyte on discharge of the cell and have to be transferred to this region on charging of the cell. The matrix however must constitute an electronic conductor to transfer electrons from the reaction zone to the cathode current collector when charging the cell and to provide the required electronic current path between the cathode current collector and the regions near the surface of the electrolyte where the sulphide ions have to be formed on discharge of the cell.
Examples of the use of graphite or vitreous carbon as a conductive matrix material in the cathode electrode of a sodium sulphur cell are shown in U.S. Pat. Nos. 3,966,492, 3,980,496, 3,985,575 and 4,002,807. As is explained in U.S. Pat. No. 3,993,503, certain advantages can be obtained by utilising two different materials, disposed in different parts of the cathodic region. One, for use in charging the cell, is preferentially wetted by the sulphur and the other, for use on discharge, is preferentially wetted by the polysulphides. For the former material, it is proposed to use graphite felt or foam on porous graphite or vitreous carbon foam or pyrolytic graphite felt or foam or other unspecified materials covered or coated with such felt or foam. The material to be preferentially wetted by the polysulphides must essentially have surface properties differing from those of carbon or graphite; they may for example be oxide materials. Such materials, in general, are even less conductive than carbon or graphite and one of the problems with the use of two different materials, one effective during charging and the other effective during discharge of the cell, is the reduction in overall conductance in the cathodic region.